清华大学：《计算机图形学基础》课程教学资源（授课教案）视图模型变换

Overview of Today • Some important concepts in Graphics – Color – Image & Pixel (象素) – Triangle Mesh – Lighting & Shading （绘制） • Transformation (变换) and Viewing

Color Perception • What is Color? – Colors are the sensations that arise from light energy of different wavelengths • electromagnetic waves (电磁波) with different wavelengths are corresponding to different colors • Human’s eye are sensitive for electromagnetic wave of wavelengths between 380 to 760 nm

Spectral Distribution of Light • ―Light‖ is a mixture of many wavelengths, each with some intensity – E.g., White Light means light include all wavelengths with the same intensity. • spectral distribution: intensity as a function of wavelength over the entire spectrum

• So, colors can be represented as such distribution function • However, use spectrum distribution for color representation is too complicated, and the corresponds is multiple-to-one. • There exits a situation as ―different spectrum distribution with the same color‖ ‖(异谱同色)

RGB Color Space • We first introduce color space for color representation. RBG is commonly used color space in Graphics – Color is represented as RGB triple (r,g,b); – People perform almost all operations on each channel separately – usually, each color value is a float value range from 0 to 1, or 0-255 if we use 8 bit integers

RGB Color Space • Color are represented as combinations of three primaries: red, green, blue) – C = rR + gG + bB – The reason why we pick red, green, and blue? • essentially because of the structure of our visual system • Our visual system is sensitive to those three colors • A demo for Color

RGB Color Space • Color are represented as combinations of three primaries: red, green, blue) – C = rR + gG + bB – The reason why we pick red, green, and blue?

RGB Color Space • Unfortunately ,Some colors cannot be written as combinations of RGB triples, because some parts of the red curve is negative

Other Color Spaces • So some other color space are also used – CMY – HSV – CIE XYZ

CMY • CMY :The other set of primaries besides RGB – Cyan(青), magenta（品红）, and yellow — complements （补色）of red, green, blue • It’s called subtractive primaries（减色系统） – RGB are additive primaries — start with black, add up to white – CMY — start with white and subtract colors from white